DEFLECTION OF LIGHT—DYSON AND OTHERS. 155 
Thus the mean value of « obtained from all the astrographic plates 
is 0.86’, a figure considerably less than that obtained from the 4-inch 
photographs. 
25. Reference to the diagram shows that the measurement of dis- 
placement depends essentially on the position of the stars Nos. 3 
and 4 relative to 5 on one side and 6 and 10 on the other. These are 
all bright stars, and in this respect their images are more comparable 
than are the images of the fainter stars. The measures of these stars 
are given in the following table: 
Measured values of Dy for stars Nos.— Measured values of Dy for stars Nos.— 
No. of No. of 
eclipse eclipse ; 
plate. 5 4 3 6 10 plate 5 4 3 6 10 
r r r T r r r Tr Tr r 
ihe og: oe —0. 051 |+0.175 |+0.169 |+0. 201 |+0. 235 |} 9--...--- —0. 059 |+0.121 |+0.109 |+-0. 205 | +0. 180 
eee Lee +0. 558 |+0.656 |+0. 724 |+0. 668 |+-0. 702 || 10..-.... +0. 033 |+0. 270 |+0.188 |+0.258 | +0. 280 
See ssacis +0. 124 |+-0. 285 |+-0. 286 }-++0. 274 |+0.355 |] 11......- +0. 025 |+-0. 215 |+0. 210 |++0. 233 | -++0. 274 
Bee sctet +0. 111 |+0. 222 |+0..247 |+0. 231 |+0.167 || 12....... —0.068 |-+0. 144 |+-0.124 |+-0.160 | +0. 167 
Tansee +0. 034 |-++0. 228 |+-0. 232 |+-0. 218 |+0.308 |] 15.....-- —0.038 |-+0.138 |+0.107 |+0.172 |.-.....- 
Geeacceacc +0. 164 |+0. 488 |+0. 478 |+0. 557 |-+0. 637 |] 16......- —0. 050 |+0.076 |+0.046 |+0.127 | +0.073 
Te Ae —0.051 |+-0.156 |+-0. 162 |++0. 250 |+0.279 |} 17......- —0. 071 |+0. 104 |+0. 081 |+0.186 | +0. 164 
Se eia ws ath +0. 108 |++-0.330 |++-0.314 |+0.376 |+0.397 |} 18......- +0.016 |+0.092 |+0.109 |+0.099 | +0. 084 
The equations given by these stars are: 
—0.160d—1.107e—0.789¢+f/=Dy, (1) 
-+-0.3834d-++0.472e+-1.336a-+7=Dy, (2) 
-++0.348d-+--0.360e+1.574a+f=Dy, (3) 
+0.587¢d-+1.099e-+0.726a+f=Dy, (4) 
+0.860d-+-1.321e-++0.589a+7=Dy,, (5) 
The mean of (4) and (5) added to (1) gives 
+0.564d-+0.103e—0.131¢4+2f=Dy,+4(Dy,+Dy,,). 
While the sum of (2) and (8) gives 
+0.682d-.0.832¢-+2.9100-+2f—=Dy,+Dy,. 
Subtracting these we get 
3.041 a-+-0.729e+-0.118d=Dy, +Dy,—Dy;—3 (Dy, +Dy,,). 
This equation has a small coefficient for e and a very small one 
for d. / 
Calculating the quantities on the right-hand side, assuming e to 
be the same for all the plates, and substituting the values of d from 
the previous table, we find: 
ie r 
a+0.240e=+0.056__--1 a+0,240e=+0.035____ 9 
a+0.240e=+0.049____2 a+0.240e=-+0.048____10 
a+0.240e=+0.047___-3 a+0.240e=-+0.045___ 11 
a+0.240e=-+0.059____4 a+0.240e=-+0.059____12 
a+0.240e=-+0.050____5 2+0.283e=-+0.026____15 
a+0.240e=-+0.059____6 a+0.240e= +0.024____16 
a+0.240e=-+ 0.036____7 a+0.240e=+0.028____17 
a+0,240e=+0.046____8 a+0.240e=+0.029____18 
